Removable polymers have found use in processing and manufacturing of electronic devices and microelectromechanical systems (MEMS) in diverse industries such as aerospace, automotive, and electronics industries. Removable polymers include certain types of inorganic polymers, organic polymers, and silicones. Depending on their properties and intended uses, removable polymers generally fall into one or more of the following categories: sacrificial polymers and reworkable polymers. Electronic devices and MEMS may be made with processes that use one or both of sacrificial and reworkable polymers.
A sacrificial polymer is a substance, such as temporary bonding adhesive and photoresist polymer, that is designed and intended for temporary use in manufacturing of electronic devices and MEMS, and by design is later at least partially removed from the devices after the substance has served its intended purpose. That is, at least some amount of the sacrificial polymer is not allowed to remain in the finished electronic device or MEMS. For example, an integrated circuit (IC) for a semiconductor device may be processed or made with alternating layers or rows of structural materials and sacrificial polymers. After a circuit is formed, the rows of sacrificial polymers are later removed and do not remain in the finished semiconductor device. Or a semiconductor wafer for a semiconductor device may be temporarily adhered to a support wafer with a sacrificial temporary bonding adhesive. The temporary bonding adhesive is later removed from the semiconductor wafer and does not remain in the finished semiconductor device.
A reworkable polymer, such as an encapsulant, interfacial layer, or pottant, is a substance that may be allowed to remain in the finished electronic device or MEMS unless a manufacturing defect is discovered therein. In the latter case, the reworkable polymer may be completely stripped from the device or MEMS in order to enable the reusing of valuable device components such as semiconductor substrates. For example, light-emitting diode (LED)-containing lamps may be made with reworkable polymer encapsulant, and if a defect in one of the lamps is discovered the encapsulant is removed from the defective lamp, and the liberated LED is re-encapsulated with new encapsulant.
Electronic device and MEMS manufacturing and operating requirements impose strict performance requirements on removable polymers. The removable polymers must be stable at temperatures up to 400° C., and yet remain readily and completely strippable using a liquid stripper that does not harm sensitive electrical or optical components. Few removable polymers and liquid strippers have found acceptance in these industries.
If incumbent removable polymers would be exposed to heat that is too high (e.g., >400° C.) during manufacturing or operating processes, it would be expected that they would suffer from one or more drawbacks. They would overly shrink, chemically degrade, lose too much mass (e.g., >15% mass loss), gain too much mass (e.g., >2% mass gain), and/or become unstrippable. The shrinkage can undesirably lead to the removable polymers pulling away from the electrical/optical component(s) with which they were in contact. The chemical degradation of incumbent removable polymers releases corrosive by-products, leading to contamination of sensitive electronic components and MEMS's. Alternatively or additionally, the chemical degradation causes removable polymers to lose properties for which they are employed such as a temporary bonding adhesive prematurely losing adhesiveness or an encapsulant losing optical transmittance properties (spectral changes or decrease in percent transmittance). The mass loss is similar to shrinkage and can undesirably result in associated changes in properties of removable polymers such as increasing hardness, decreasing flexibility, or decreasing thermal conductivity/insulation. The effects of mass gain are similar to the contamination or property loss effects of chemical degradation. Overheating the removable polymers can undesirably lead to their inertness towards liquid strippers, making processing and manufacturing operations inefficient or inoperable.
The nature of the compositions of today's incumbent removable polymers, however, limit their useful manufacturing and operating conditions to a maximum temperature of about 400° C., and typically not more than 300° to 350° C. Above this temperature, it would be expected that incumbent removable polymers would suffer from one or more of the aforementioned drawbacks. For example, a common problem that prevents using incumbent removable polymers at a temperature above 400° C. is that they lose more than 15% mass and/or become unstrippable in liquid strippers. Above 400° C., other incumbent removable polymers undergo chemical degradation and/or gain mass.
The operating limitations of incumbent organic polymers are illustrated by polyimide, which is said to have a maximum service temperature of about 400° C. Polyimide becomes unstable at temperatures above 400° C., where it degrades and gain or lose mass. It may also become unstrippable.
The operating limitations of incumbent inorganic polymers are illustrated by the cured trialkoxaysilane hydrolysates of U.S. Pat. No. 5,762,697 to Yoshinori Sakamoto et al. (SAKAMOTO). SAKAMOTO's polymeric inorganic silsesquioxanes lack or are free of organic groups (e.g., hydrocarbyl groups). The cured trialkoxysilane hydrolysates, which are made by hydrolyzing a trialkoxysilane of formula (HSi(O(C1-C4)alkyl)3), are said to exhibit a very unique thermogravimetric behavior when the thermogravimetric analysis is undertaken in air. Namely, the thermogravimetric curve taken by increasing the temperature [in air] indicates an increase [of weight] of several % (e.g., 3%) at a temperature above 300° C. in great contrast to a conventional coating solution containing a trialkoxy silane hydrolysate which indicates a weight decrease in the thermogravimetric analysis. Films of the cured trialkoxysilane hydrolysates were baked in air at 400° C. for 30 minutes (Application Examples 1 to 3), and then subjected to a test of gas evolution by thermal decomposition using a gas evolution tester and increasing the temperature from 50° to 600° C. Degradation evidenced by gas evolution was observed. Additionally, one of the trialkoxysilane hydrolysate prepolymers and a comparative trialkoxysilane hydrolysate prepolymer were subjected to the thermogravimetric analysis (TG) and differential thermal analysis (DTA) in the temperature range of 40° to 800° C. at a temperature elevation rate of 10° C./minute (Application Example 4) to give solid material 1 and 2, respectively. The solid material 1 indicated a weight increase of about 4.0% at 360° C. or higher up to 800° C., while the solid material 2 indicated a weight decrease of 9.3% when heated up to 800° C. SAKAMOTO's polymeric inorganic silsesquioxanes are vulnerable to excessive weight gain or excessive weight loss at high temperatures and may be unstrippable.
Silicones, with their silicon-bonded organic groups (e.g., hydrocarbyl groups), would be expected to be less mass stable at temperatures above 450° C. than the inorganic polymers of SAKAMOTO. This is because the silicon-bonded organic groups of silicones contain carbon-hydrogen, carbon-carbon, and carbon-silicon bonds, whereas the inorganic polymers of SAKAMOTO do not. These carbon-hydrogen, carbon-carbon, and carbon-silicon bonds provide extra ways for the silicones to undergo chemical decomposition at high temperatures. Examples of such chemical decomposition are reactions that result in oxidation (mass gain), gas evolution, depolymerization, random chain scission, side-group elimination, or a combination of any two or more thereof. These chemical degradations would render the silicones unsuitable for their intended use.
The operating limitations of incumbent silicones are illustrated by incumbent polymeric silsesquioxanes. For example, heating the polymeric silsesquioxane resins mentioned in U.S. Pat. No. 5,010,159 to Howard M. Bank et al. (BANK) and in U.S. Pat. No. 5,063,267 to Larry F. Hanneman (HANNEMAN) above 350° C. may give a polymer that is not strippable. BANK's and HANNEMAN's polymeric silsesquioxane resins also tend to lose too much mass when heated above 400° C. Further, curing BANK's and HANNEMAN's silsesquioxane resins (prepolymers) from silane monomers undesirably requires heating at high temperatures (e.g., 300° or 350° C.). The high temperatures increases process limitations and costs for applications manufacturers of these silsesquioxane resins on devices.
Further illustration of the operating limitations of incumbent polymeric silsesquioxanes is found in the antireflective coatings mentioned in U.S. Pat. No. 7,756,384 B2 to Peng-Fei Fu, et al. (“FU #1”; that Peng-Fei Fu is also a present inventor). FU #1 generally mentions curing silsesquioxane resins (prepolymers) at a temperature of from 80° to 450° C. for 0.1 to 60 minutes to give polymeric silsesquioxanes. In actual practice, certain of FU #1's examples mention baking the silsesquioxane resins (prepolymers) at a maximum temperature of 350° C., but the resulting polymeric silsesquioxanes could not be stripped with a commercial wet stripping solution ACT(R) NE-89 or CC1. Baking the silsesquioxane resins above 350° C. would only worsen the problem.
Further illustration of the operating limitations of incumbent polymeric silsesquioxanes is found in the antireflective coatings mentioned in U.S. Pat. No. 8,653,217 B2 to Peng-Fei Fu, et al. (FU #2; again, that Peng-Fei Fu is also a present inventor). FU #2 generally mentions curing silsesquioxane resins (prepolymers) at a temperature from 80° to 450° C. for 0.1 to 60 minutes to give polymeric silsesquioxanes. In actual practice, certain of FU #2's examples mention baking the silsesquioxane resins (prepolymers) at a maximum temperature of 250° C. The resulting polymeric silsesquioxanes could be completely stripped with ACT(R) NE-89 or CC1. Based on FU #1, however, baking the silsesquioxane resins of FU #2 above 350° C. would be expected to give polymeric silsesquioxanes that could not be stripped with ACT(R) NE-89 (Etch Residue Remover, Air Products and Chemicals, Inc.) or CC1 (Contact Clean 1, ATMI).